Photovoltaics is concerned with direct conversion of light or solar energy into electricity through the use of active electronic devices called solar cells. Solar cells are commonly fabricated on wafers of silicon. However, the cost of electricity generated using silicon-based solar cells is rather high compared with the cost of electricity generated by traditional methods. One way to make photovoltaics competitive with traditional methods of electric power generation is to develop low cost thin film solar cells. This, in turn, requires the development of film growth techniques that can deposit electronically active layers of absorber materials and other components of the solar cells on large area substrates, using cost effective approaches with high materials utilization.
Group IB-IIIA-VIA materials are considered to be highly promising as the absorber layers of high efficiency thin film solar cells. In fact, a comparatively high efficiency thin film device with a conversion efficiency of over 17% has already been produced on a Cu(In,Ga)Se.sub.2 absorber film grown by a vacuum evaporation technique.
The electrical and optical properties of Group IB-IIIA-VIA compound films depend on their chemical composition, defect chemistry and structure, which in turn are strongly related to the film growth techniques and parameters. There are a variety of deposition techniques that have been used for the growth of Group IB-IIIA-VIA compound semiconductor films. However, it is crucial to obtain a material that has the good opto-electronic and structural properties which are needed for the production of active electronic devices such as solar cells.
In solar cells based on a Group IB-IIIA-VIA absorber film, appreciable amounts of the binary phases such as Group IIIA-VIA compounds and especially Group IB-VIA compounds in the absorber layer, typically deteriorate the electronic properties of the compound, and thus the characteristics of the solar cells. In addition, it is considered desirable to have an absorber material with columnar grains equivalent to at least about 0.5 .mu.m diameter, in thin film solar cell structures. Furthermore, for commercial viability, the deposition technique employed should be able to deposit a layer that has a relatively uniform composition onto very large substrates, such as several ft.sup.2 in area, using low cost equipment and processes.
An important compositional parameter of Group IB-IIIA-VIA thin films is the molar ratio of the Group IB element or elements to the Group IIIA element or elements. This is commonly referred to as the I/III ratio. Typically an acceptable range of the I/III molar ratio for the Cu-containing Group IB-IIIA-VIA absorber is about 0.8-1.0, although in some cases involving doping with a dopant such as Na, this ratio can go even lower to about 0.6. If the I/III ratio exceeds 1.0 in any part of the absorber film, low resistivity Group IB-VIA phases typically precipitate and deteriorate the performance of the device.
One technique that has yielded relatively good quality Group IB-IIIA-VIA films for solar cell fabrication is co-evaporation of Group IB, IIIA and VIA elements onto heated substrates. As described by Bloss et al. in their review article ("Thin Film Solar Cells", Progress in Photovoltaics, vol. 3, page 3-24, 1995), the film growth in this technique takes place in a high vacuum chamber and the evaporation rates of the Group IB and Group IIIA elements are carefully controlled to keep the overall I/III ratio of the film in the acceptable range.
However, the evaporation method is not readily adaptable to low cost production of large area films, mainly because uniform deposition by evaporation on large area substrates is difficult, and the cost of vacuum equipment is high.
Another technique for growing Group IB-IIIA-VIA compound thin films for solar cells is a two-stage process where at least two components of the Group IB-IIIA-VIA material are first deposited on a substrate, and then reacted with each other and/or with a reactive atmosphere in a high temperature annealing process. U.S. Pat. No. 4,581,108 issued to Vijay K. Kapur et al., in 1986, U.S. Pat. No. 4,798,660 issued to James H. Ermer et al., in 1989, and U.S. Pat. No. 5,028,274 issued to Bulent M. Basol et al., in 1991 teach respectively the methods of electrodeposition of Group IB and IIIA elements onto a substrate followed by selenization or sulfidation, DC magnetron sputtering of Cu and In layers on a substrate followed by selenization, and deposition of Group IB and IIIA elements onto a substrate previously coated with a thin Te film followed by selenization or sulfidation. The initial layers deposited on the substrate before the selenization or sulfidation heat treatment steps are commonly referred to as precursor films or layers.
In the two-stage processes, large area magnetron sputtering techniques can be used to deposit individual layers containing Group IB and Group IIIA elements for precursor film preparation. In the case of CuInSe.sub.2 growth for example, Cu and In layers can be sputter deposited on non-heated substrates and then the resulting precursor can be selenized in H.sub.2 Se gas or Se vapor at an elevated temperature, as is shown in U.S. Pat. Nos. 4,798,660 and 5,028,274.
The techniques employed for the growth of Group IB-IIIA-VIA films require strict control of the material composition during the deposition process, with a typical goal that in the final film, the overall I/III ratio should be in the acceptable range of about 0.80-1.0. For mass production of photovoltaic modules, this ratio should be uniform over large area substrates. Therefore, in the two-stage processes involving deposition of consecutive layers containing the Group IB element and the Group IIIA element or elements, the uniformity and the thickness of each deposited layer has to be controlled.
When the I/III ratio exceeds 1.0, it causes the separation of, e.g., a Cu-sulfide, selenide or telluride phase in exemplary Group IB-IIIA-VIA compound layers where the Group IB element is Cu. Layers containing these phases have low resistivities and typically are not used in active device fabrication. However, these Cu-rich films have good structural characteristics and large grain sizes. The relationship between the structural properties of Group IB-IIIA-VIA materials and their composition can be used beneficially, especially in the co-evaporation approaches, by intentionally increasing the I/III ratio above 1.0 during the film growth process for improving the structural properties of the growing film, and then decreasing it back to the acceptable range by the time deposition process is terminated. Films grown by such approaches often have large grain sizes and good electronic properties. Therefore, it is typically allowable to change the I/III ratio during the deposition and growth of a Group IB-IIIA-VIA compound, but with the overall ratio in the final film being within the 0.8-1.0 range.
Since the uniformity and control of the I/III ratio throughout the film is important for Group IB-IIIA-VIA compounds, attempts have been made to fix this ratio in a material, before the deposition process, and then transfer this fixed composition into the thin film formed using the material. One such attempt to prepare Group IB-IIIA-VIA compound films using a material with a pre-fixed composition was screen printing layers onto substrates and their conversion into the compound. T. Arita et al. in their 1988 publication (20th IEEE PV Specialists Conference, 1988, page 1650) described a screen printing technique that involved: creating an initial material by mixing pure Cu, In and Se powders in the compositional ratio of 1:1:2, milling these powders in a ball mill and forming a screen printable paste, screen printing the paste on a substrate, and sintering this precursor film to form the compound layer. The milling was done in a solvent such as water or ethylene glycol monophenyl ether to reduce the particle size, and formation of a paste was done using a propylene glycol binder. The paste material was deposited on a high temperature borosilicate glass substrate by the screen printing method, forming a film. The post-deposition treatment step consisted of annealing the film in nitrogen gas at 700.degree. C., to form a compound film on the substrate.
For evaluating the photovoltaic characteristics of the resulting compound, thick pellets were made from the material obtained as a result of the milling and sintering steps, and solar cells were fabricated on them. Efficiencies of only about 1% were reported for these devices. The sintering temperature of 700.degree. C. is very high. Such a temperature is expected to cause In loss through vaporization. It would also deform the soda-lime glass substrates used in low cost solar cell structures.
Thin layers of CuInSe.sub.2 deposited by a screen printing method were also reported by a research group at Ghent State University in Belgium. A. Vervaet et al., in their 1989 publication (9th European Communities PV Solar Energy Conference, 1989, page 480), referring to the work of T. Arita et al., indicated that indium powder easily oxidizes, giving rise to unwanted phases, such as In(OH).sub.3 or In.sub.2 O.sub.3 in the final films. The technique of the Ghent research group therefore, employed the steps of: forming a CuInSe.sub.2 powder as an initial material by crushing a CuInSe.sub.2 ingot; grinding the CuInSe.sub.2 powder in a ball mill; adding excess Se powder and other agents such as 1,2-propanediol into the formulation to prepare a screen printable paste; screen printing layers onto borosilicate and alumina substrates; and high temperature sintering of the layers (above 500.degree. C.) to form the compound films. A difficulty in this approach was finding a suitable sintering aid or fluxing agent for CuInSe.sub.2 film formation. Among many agents studied, copper selenide was the best for grain growth, but films containing this phase could not be used for active device fabrication since they had I/III ratios larger than 1.0.
More recently, the Ghent group experimented with CuTlSe.sub.2, a compound with a relatively low (about 400.degree. C.) melting point, as a fluxing agent. In their 1994 publication (12th European PV Solar Energy Conference, 1994, page 604) M. Casteleyn et al., used CuTlSe.sub.2 in their formulation of the CuInSe.sub.2 paste, and demonstrated grain growth for films with I/III ratios in the acceptable range. However, the solar cells fabricated on the resulting layers were still poor with conversion efficiencies of only of about 1%. The sintering temperature of above 600.degree. C. used in this process was also high for low cost glass substrates.
Conversion of Group IB-IIIA oxide films into Group IB-IIIA selenide or sulfide layers by reaction in a selenizing or sulfidizing atmosphere has also been investigated. S. Weng and M. Cocivera (Journal of Applied Physics, vol. 74, p. 2046, 1993) and M. E. Beck and M. Cocivera (Thin Solid Films, vol. 272, p. 71, 1996) first formed a copper-indium-oxide film on a substrate by spray pyrolysis of an aqueous solution containing indium-nitrate and copper-nitrate, and subsequently reacted this layer with selenium vapor for up to 12 hours at 400-450.degree. C. to form CuInSe.sub.2. They observed a material utilization of 12.5-35% and slight loss of In upon selenization of samples. The resistivities of the p-type films were 1-10 ohm-cm. Apparently, no solar cells were fabricated on these layers. The cited resistivities are low and the material utilization is poor for large scale deposition of films with this specific technique. Reaction times are also prohibitively long. More importantly, as reported by the authors, the I/III ratio control is not very good.
Conversion of oxide films into chalcopyrite layers is also reported in U.S. Pat. No. 5,445,847, issued to T. Wada et al., in 1995. These researchers indicate that in production methods wherein the two stacked layers of the Group IB metal and of the Group IIIA metal are treated with heat under the presence of the chalcogen to obtain a chalcopyrite-type compound, there is a problem in that a deviation is observed in the I/III ratio in the obtained compound, and that the composition itself is not always microscopically constant. The researchers explained their observation by referring to the low melting temperatures of Group IIIA metals. They indicated that when laminated thin films of the Group IB metal and the Group IIIA metal are treated under a chalcogen atmosphere including, for instance, selenium or sulfur, or with a chalcogen-containing gas like H.sub.2 Se, CS.sub.2 or H.sub.2 S, along with heat, the film of the Group IIIA metal is molten and forms a great number of liquid drops resulting in a heterogeneous layer. As a remedy to this problem Wada et al. used a Group IB-IIIA oxide composition which has a high melting temperature, instead of the metallic layers. They concluded that the Group IB-IIIA oxide composition does not melt from the heat treatment temperature under a reducing atmosphere containing the Group VIA element, and that the initial composition can be maintained in micro-scale. In their reported process, the oxygen atoms contained in the oxide composition are removed in the reducing atmosphere containing the Group VIA element or elements and the oxygen atoms are, at the same time, as part of a single operation, substituted by the atoms of the Group VIA element or elements, avoiding the melting of the Group IIIA metal. This way the chalcopyrite-type compound is synthesized.
More recently, the same research group described a method of including dopants in sputter deposited Group IB-IIIA oxide layers and their conversion into Group IB-IIIA-VIA compounds through selenization (T. Negami et al., U.S. Pat. No. 5,728,231, issued Mar. 17, 1998).
Two-stage processes that involve selenization or sulfidation of Group IB/Group IIIA metallic stacks or Group IB-IIIA alloy layers are known to yield high quality Group IB-IIIA-VIA compounds for device applications. The challenge, however, in employing the two-stage processes in large scale deposition of Group IB-IIIA-VIA films is to control the I/III ratio on large area substrates. In U.S. Pat. No. 4,581,108, for example, electrodeposition was used to deposit Cu and In layers on glass/Mo (glass coated with Mo) substrates. The resulting Cu/In stacked layers were then selenized to obtain CuInSe.sub.2 compound layers. To be able to control the Cu/In ratio over large substrates in such a technique requires the control of individual thicknesses of the Cu and In layers, which is difficult to do on large areas by the electroplating approach. U.S. Pat. Nos. 4,798,660 and 5,028,274 use magnetron sputtering or evaporation approaches for the deposition of Cu and In layers which form Cu--In alloy films on the glass/Mo substrates. These alloy films are then selenized to form the desired CuInSe.sub.2 compound. Although better suited for large area deposition, the magnetron sputtering technique also needs to be closely controlled to assure a fixed Cu/In ratio over large area substrates. This is very expensive.
In a Final Technical Report, "Deposition of Copper Indium Diselenide Films by Low-Cost Techniques", February, 1987 (by Poly Solar Inc. under SERI subcontract XL-4-03125-1), the researchers in the report explained how they attempted to thermally reduce Cu--In compound films (including oxygen) to form Cu--In alloy layers and then to selenize these metallic alloys to form CuInSe.sub.2. They started with the premise that in order to deposit uniform Cu films they had to apply a copper compound to the substrate in the form of a solution. Therefore, since CuO and Cu.sub.2 O were insoluble in water, they used Cu.sub.2 O and formed a solution in ammonium hydroxide. In.sub.2 O.sub.3 was collodially suspended in this solution since it was not soluble. The mixture was then placed on a substrate and chemical reduction was carried out in a hydrogen flow at a substrate temperature of 550.degree. C. By optimizing the process parameters, uniform Cu--In films were reportedly obtained. However, the In content in the films was lower than expected in all cases indicating loss of In during processing. Further, the ammonia solution of Cu.sub.2 O was found to be unstable over a period of several days. Therefore, the researchers reported that they discontinued this processing approach.
The above researchers reported another experiment that involved dissolving Cu(NO.sub.3).sub.2 and In(NO.sub.3).sub.3 in methanol and depositing this solution onto a substrate. After drying, the substrate was annealed in hydrogen atmosphere at 550.degree. C. to obtain Cu--In films. The films obtained this way were also found to be In-deficient due to vaporization of In-oxide or In. Therefore, the researchers adjusted the initial stoichiometry to more In-rich compositions so that they could obtain films with Cu/In ratios close to 1.0 after processing. Cu--In films were then selenized in an H.sub.2 +H.sub.2 Se gas mixture and Cu--In--Se films were obtained. In conclusion, the researchers pointed out two major problems with the techniques they attempted to develop: composition inhomogeneities and the irreproducible resistivity values. It should be noted that these are two of the most important parameters that need to be controlled in a Group IB-IIIA-VIA deposition technique. Solar cells fabricated on the absorbers produced, showed only very weak photo response and an efficiency of less than 1%.
As the above review demonstrates, there is a need for techniques to provide Group IB-IIIA-VIA and related compound films on large area substrates, with good compositional control and uniformity. There is also a need for such compound films with superior electronic properties, that would make them suitable for the fabrication of active electronic devices such as solar cells with conversion efficiencies of close to or above 10%.